During the transit of an exoplanet across a rotating star, it selectively hides different portions of the stellar surface, where the local rotational velocity vector is toward or away from the observer. Thus, part of the blue- or redshifted photons are removed from the integrated stellar flux, whose averaged wavelength then appears slightly red- or blueshifted. The effect has been measured for numerous exoplanets, where measurements of this Rossiter-McLaughlin effect enable determining the geometry of the projected path of the exoplanet [1]}, [2]}, [3]}.
